Ocular surface disease in dogs part 1: aetiopathogenesis and clinical signs

Dry eye disease (DED) is a commonly diagnosed disorder in small animals. It has been historically described as a quantitative or qualitative deficiency in tear production; however, the pathogenesis of the disease is much more complex than previously recognised. Ocular surface disorders in people are currently investigated as a dysfunction of what is termed the ‘lacrimal functional unit’, consisting of the ocular surface, including Meibomian glands and conjunctiva, in addition to lacrimal glands, eyelids and sensory and motor innervation. A loss of homeostasis of the tear film leads to the vicious cycle of increased osmolarity and inflammation, with dreadful consequences for the ocular surface in both dogs and humans. Ocular inflammation is the hallmark of DED, reflected in early nonspecific conjunctivitis and later in dramatic changes of the ocular surface. Prompt recognition of the disease, accurate diagnostics and appropriate treatment are paramount in preserving the ocular surface integrity and the patient's comfort. This article discusses the aetiopathogenesis of the condition, and describes the clinical signs, while the second article will look at diagnostics and treatment of ocular surface disorders in dogs.